1. Field of the Invention
The present invention is related to cargo container lifting apparatus, and more particularly to the operation of the twist locks of a cargo container lifting spreader, and still more particularly to means for interlocking the twist lock control switch to prevent improper operation thereof.
2. Description of the Prior Art
Cargo containers, which are standardized in size for ease in handling, have in recent years come into widespread use in the shipping industry. Such containers are conventionally handled by lifting cranes which pick up containers with a so-called lifting spreader which is a load spreading lifting device conforming in shape to the top of the container. The spreader locks onto the container by means of corner twist locks which must be aligned and dropped into twist lock receptacle castings built into each of the container's top corners. To move a container, the crane operator first positions the spreader over the container to be lifted such that the twist locks are lined up over the container's corner castings; he then lowers the spreader onto the top of the container whereupon a manual control switch is thrown to rotate the twist lock into a locked position. Similarly, to disengage a spreader from a container which has already been moved, the operator first throws the same control switch to unlock the twist locks and then lifts the spreader away from the container.
Problems arise, however, when the lifting spreader is not properly engaged with the container before the operator turns the manual control switch to close the twist locks. If this happens, which it often does, the twist locks may be rotated against a structural part of the container resulting in damage to the twist lock or the container or both. This situation often occurs because of operator haste which itself is often motivated by a pay formula based on the number of containers an operator handles.
To prevent premature rotation of the twist lock and the resulting equipment damage, interlock systems have been devised to permit rotation of the twist lock only upon proper engagement of the spreader. Conventionally, such interlocks are comprised of a mechanical pressure cam disposed next to each twist lock such that the cam will be depressed to release its associated twist lock when the lifting spreader contacts the top of the container. Such mechanical interlocks, however, have reliability problems and are not suitably adapted to interface with other electrical circuitry of the crane control system. To provide for electrical interlocking of the twist locks, an electrical interlock system has been devised utilizing proximity sensors at the twist locks for sensing the proximity between the lifting spreader and the container. This known system uses, in place of the mechanical cams, proximity switches which are series connected, normally open switches which provide a relay energizing current when all the proximity switches are closed. Upon closing of all the proximity switches of the system, power is supplied to the operator's twist lock manual control switch. It has been found, however, that this electrical interlock system operates irratically, in that, quite often the last in the series of sensors would not operate properly, thereby preventing proper operation of the overall system. It also does not provide the important visual and audio operation controls as will be described in the present invention.
The present invention is an improvement on the above-described electrical interlock system, in that, it overcomes the problem of reliability in respect to sensor operation. It also provides a solution to the over-eagerness problem which occurs with many container crane operators as they proceed through the container lifting operations, namely, engaging a container with a lifting spreader, moving the container to its new location, setting the container back down, and finally disengaging from the container.